Nuclear RNA surveillance in Saccharomyces cerevisiae: Trf4p-dependent polyadenylation of nascent hypomethylated tRNA and an aberrant form of 5S rRNA.

1-Methyladenosine modification at position 58 of tRNA is catalyzed by a two-subunit methyltransferase composed of Trm6p and Trm61p in Saccharomyces cerevisiae. Initiator tRNA (tRNAi(Met)) lacking m1A58 (hypomethylated) is rendered unstable through the cooperative function of the poly(A) polymerases, Trf4p/Trf5p, and the nuclear exosome. We provide evidence that a catalytically active Trf4p poly(A) polymerase is required for polyadenylation of hypomethylated tRNAi(Met) in vivo. DNA sequence analysis of tRNAi(Met) cDNAs and Northern hybridizations of poly(A)+ RNA provide evidence that nascent pre-tRNAi(Met) transcripts are targeted for polyadenylation and degradation. We determined that a mutant U6 snRNA and an aberrant form of 5S rRNA are stabilized in the absence of Trf4p, supporting that Trf4p facilitated RNA surveillance is a global process that stretches beyond hypomethylated tRNAi(Met). We conclude that an array of RNA polymerase III transcripts are targeted for Trf4p/ Trf5p-dependent polyadenylation and turnover to eliminate mutant and variant forms of normally stable RNAs.